a sub-aerial habitat subject only to such occasional supphes
of water as is afforded by the rainfall. Of this nature are
some of the species of Vaucheria. A very few species, like
Chroolepus, which grows on rock surfaces, are comparable
with the land plants which have been termed xerophilous.
Plankton.
The great majority of the aquatic algae, both freshwater and
marine, are attached plants. Some, however, are wanderers,
either swimming actively with the aid of cilia, or floating
inertly as the result of a specific weight closely approaching
that of the medium. To the aggregate of such forms, both
animal and vegetable, the term plankton has been applied, and
the investigation of the vegetable plankton, both freshwater
and marine, has been pursued in recent times with energy and
success. The German Plankton Expedition of 1889 added greatly
to our knowledge of the floating vegetable life of the North
Atlantic Ocean, while many laboratories established on the
shores of inland seas and lakes have rendered a similar
service in the case of our freshwater phyto-plankton.
The quantitative estimate of the amount of this flora has
revealed its enormous aggregate amount and therefore its great
importance in the economy of oceanic and lacustrine animal
life. The organisms constituting this plankton are mostly
unicellular, often aggregated together in colonies, and the
remarkable structure which they exhibit has added a new chapter
to the story of adaptation to environment. The families
Diatomaceae, Peridiniaceae and Protococcaceae are best
represented in the pelagic plankton, while in addition the
Volvocaceae are an important element in freshwater plankton.
Benthos.
The great majority of algae, however, grow like land-plants
attached to a substratum, and to these the term benthos is
now generally applied. While the root of land-plants serves
for the double purpose of attachment and the supply of water,
it is attachment only that is usually sought in the case of
algae. Immersed as they usually are in a medium containing in
solution the inorganic substances which they require for their
nutrition, the absorption of these takes place throughout
their whole extent. The elaborate provision for the conduct
of water from part to part which has played so important
a role in the morphological development of land plants is
entirely wanting in algae, such conducting tissues as do
exist in the larger Phaeophyceae and Rhodophyceae serving
rather for the convection of elaborated organic substance, and
being thus comparable with the phloem of the higher plants.
The attachment organ of algae is thus more properly called
a holdfast, and is found to be of very varied structure.
It generally takes the form of a single flattened disc as
in the Fucaceae, or a group of finger- like processes as in
Laminariaceae, or a tuft of filaments as in many instances.
When the attachment is in sand or mud, it often simulates the
appearance of a true root as in Chara or Caulerpa. It is
clear that where the bottom of a lake or sea consists of oozy
mud or shifting sand, it is impossible for algae to secure a
foothold. Thus a rock emerging from a sandy beach may often
be observed to stand covered with vegetation like an oasis in a
desert. The rapidity with which walls, piles and pontoons--stone,
wood and iron--become covered with marine plants is well known,
while the discovery of some effective means of preventing
the fouling of the bottoms of ships by the growth of algae
would be hailed as a boon by shipowners. While rocks and
boulders are the favoured situation for the growth of marine
algae, those which readily disintegrate, like the coarser
sandstones, are naturally less favoured than the hard and
resistant. A large number of algae again live as epiphytes or
endophytes. In the case of the freshwater species the host-plants
are mostly species of aquatic Graminaceae, Naiadaceae or
Nymphaeaceae. In the case of marine algae, the hosts are
chiefly the larger Phaeophyceae and Rhodophyceae. A bed of
Zostera near the level of low water is, however, on the
British coast a favourite collecting ground for the smaller
red and brown epiphytes. Of endophytes a distinction must be
made between those which occupy the cell-wall only and those
which perforate the cells, bringing about their destruction.
There can be little doubt that in some cases the epiphytism
approaches parasitism. In one case described by Kuckuck the
chromaphores of the infesting algae are absent, a circumstance
which points to a complete parasitism. Allusion has already
been made to the peculiar habit of the shell-boring algae.
Habit.
In many algae certain branches of limited growth bear a remarkable
resemblance to leaves. The Characeae among freshwater algae
and the Sargassaceae among marine algae might be cited as
examples. Surveying the whole range of algae life, Oltmanns
distinguishes bush-forms, whip- forms, net-forms, leaf-forms,
sack-forms, dorsi-ventral forms, and cushions, plates and
crusts. The similarity of outline in many species to that
of trees and shrubs will strike any one who examines algae
mounted for the herbarium. Cladophora and Bryopsis among
monosiphonous forms, Chara, Polysiphonia, Ceramium and
Cystoseira among larger algae, are illustrations of this.
The whip-forms are represented by Spirogyra, Chaetomorpha,
Scytosiphon, Nemalion, Himanthalia and Chorda. Net-forms are
found in Hydrodictyon and Microdictyon. The leaf-forms are
very varied and owe their existence to the advantage accruing
from the exposure of a large surface to the influence of the
light. In some cases such as Delesseria, Neurymenia, Fucus,
Alaria, the leaf-like structure is provided with a strengthening
mid-rib, and when as in Delesseria it is also richly veined
the resemblance to the leaf of a flowering plant is striking.
Laminaria, Padina, Cutleria, Punctaria, Iridaea, Ulva,
Porphyra, are leaf-like with a rigidity varying from a fleshy
lamina to the thin and pliable. Agarum, Claudea and Struvea
are leaf-forms which are perforated like Aldrovanda among
flowering plants. Enteromorpha, Asperococcus and Adenocystis
are sack-forms. Dorsi-ventral algae are rare. Leveillea
jungermanneoides bears a remarkable resemblance to a leafy
liverwort. In the next group of forms the simplest are crusts
attached to the substratum throughout their extent, and growing
at the margin. Such are Myrionema, Ralfsia, Melobesia and
Hildebrandtia. Others are attached throughout their extent,
but also grow vertical filaments so as to form a velvety pile.
Such are Coleochaete, Ochlochaete, Elachistea, Ascocyclus and
Rhododermis. Peysonellia squamaria, Melobesia lichenoides,
Leathesia difformis are forms which are not attached
throughout but grow in plates like the foliaceous lichens.
Ecology.
When it is sought to consider algae with a view to the
correlation of the external form to the conditions of life,
a subject the study of which under the name of ecology has
been latterly pursued with great success among land plants,
it is difficult as yet to arrive at generalizations which are
trustworthy. Among land plants, as is well known, similarity
of environment has often called forth similar adaptations
among plants of widely separated families. The similarity
of certain xerophilous Euphorbiaceae to Cactaceae is a ready
illustration of this phenomenon. From what has been already
said it is evident that among algae also strikingly similar
forms exist in widely different groups. Instances might be
multiplied. Compare, for example, the blue-green Gloeocapsa
with the green Gloeocystis, the red Batrachospermum
with the green Draparnaldia, the red Corallina with the
green Cymopolia, the green Enteromorpha with the brown
Asperococcus, the green Ulva with the red Porphyra, the
red Nemalion with the brown Castagnea, and so on. But on
the one hand similar forms seem to grow often under different
conditions, while on the other hand different forms flourish
under the same conditions. The conceivable variations in the
conditions which would count in algal life are variations in
the chemical character of the water--whether fresh, brackish
or salt; or in the rate of movement of the water, whether
relatively quiet, or a stream or a surf; or in the degree of
illumination with the depth and transparency of the water.
But the laws which determine the associations of various
algae under one environment are as yet little understood. The
occurrence of a plentiful mucilage in many freshwater forms
is, however, doubtless a provision against desiccation on
exposure. The fine subdivision of filamentous and net-forms is
similarly a provision for easy access of water and light to all
parts. The calcareous deposits in Characeae, Corallinaceae
and Siphonaceae are at once a protection against attack and
a means of support. The whip-forms would seem to be designed
to resist injury from surf or current. The vesicles of
Fucaceae and Laminariaceae prevent the sinking of the bulkier
forms. But why certain Fucaceae favour certain zones in
the littoral region, why certain epiphytes are confined
to certain hosts, why Red and Brown Algae are not better
represented in fresh water Or Green Algae in salt,--these
are problems to which it is difficult to find a ready answer.
Uses.
Algae cannot be regarded as directly important in the
industries. On the coasts of Europe marine algae detached
by the autumnal gales are commonly carted on to the land as
a convenient manure. Porphyra laciniata and Rhodymenia
palmata are locally used as food, the latter being known as
dulse. Agar-agar is a gelatinous substance derived from an
eastern species of Gracilaria. The ash of seaweeds, known in
Scotland as kelp, and in Brittany as varec, was formerly used as a
source of iodine to a greater extent than is at present the case.
Occurence in the rocks.
Excepting where the thallus is impregnated with silica, as
in Diatomaceae, or carbonate of lime, as in Corallinaceae,
Characeae and some Siphonales, it is perhaps not surprising that
algae should not have been extensively preserved in the fossil
form. Considering, however, that it is generally believed
that Bryophyta and vascular plants are descended from an
algal ancestry, it is natural to suppose that, prior to the
luxuriant vegetable growths of the Carboniferous period,
there must have existed an age of algae. It was doubtless
this expectation that has led to the description of a number
of Silurian and Devonian remains as algae upon what is now
regarded as inadequate evidence. The geologic record is,
as perhaps is to be expected, exceedingly poor, except as
regards the calcareous Siphonales, which are well represented
at various horizons, from the Silurian to the Tertiary;
even the Diatomaceae, which are found in great quantities in
the Tertiary deposits, do not occur at all earlier than the
chalk. It is believed, however, that the Devonian fossil,
Nematophycus, is a Laminarian alga, but it is not until
the late Secondary and the Tertiary formations that fossil
remains of algae become frequent. (See PALAEOBOTANY.)
The subjoined list includes the larger standard works on
algae, together with a number of papers to which reference is
made in this article. For a detailed catalogue of Algological
literature, see the ``Bibliotheca Phycologica'' in de Tonii's
Syllope Algarum, vo1. i. (1889), with the addendum thereto
in vol. iv. (1897) of the same work. GENERAL.--J. G. Agardh,
Species, genera et ordines Algarum (vols. i-iii., Algernes
Systematik (Lund, 1872-1899); J. E. Areschoug, ``Observationes
